The present invention relates to improvements in an up-shift control apparatus of an automatic transmission, and more particularly to a control of an accumulator back pressure of a friction element to be disengaged in an up-shift transient time.
Conventionally, an up-shift operation of an automatic transmission is executed such that when a charging pressure of an engaged friction element reaches a predetermined pressure value, an accumulator back pressure of a disengaged clutch is drained through a timing control by means of a switching control of a sequence valve so as to decrease the hydraulic capacity of the clutch discharging pressure. Thereafter, the shift change is finished. In this up-shift, if a throttle opening degree of an engine is small during the shift change, a shock due to the shift change is sufficiently suppressed. However, if the throttle opening degree becomes within a range from medium to high during the shift change, the change of the engine rotation speed due to the shift change becomes large and the change of the rotation due to the inertia is applied to the output shaft. Therefore, the output shaft torque is largely decrease at an ending period of the shift change, and large chock is generated.
In order to decrease such shock at the ending period during the up-shift, Japanese Provisional Publication No. 2-229959 discloses a method for decreasing the change of the engine rotation speed. More particularly, when an actual speed ratio becomes a speed ratio indicative of a period just before the end of an inertia phase, a charging pressure of a friction element to be engaged is temporally decreased so as to delay the proceeding of the shift change for the purpose of the decreasing of the change of the engine rotation speed.
However, this conventional up-shift control apparatus has problems such that a period for the shift change is elongated by sufficiently ensuring the suppression of the shift change shock or that the shift change shock is generated by an insufficient time elongation. That is, in case that the up-shift is execute through only the charging pressure decreasing control, the magnitude of the decreased charging pressure is set so as to suppress the elongation of the period for the shift change. Therefore, it is difficult to preferably suppress the engagement shock by this conventional control method.